This application pertains to the art of data communication, and more particularly to the art of cyclic data transfers among data equipment. The present invention is particularly applicable to a loop data transmission system which is suited for realizing high-speed data communication among computers or peripherals, and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications such as communication of data between a computer and a peripheral, or among peripherals.
In a prior loop transmission system, a token passing system has predominately been adopted. In such a "token ring" system, frame information for assuring the priority of data transmission is circulated along the transmission line in groups of data often referred to as "frames." A frame with priority is allowed to conduct and maintain data communication between devices, thereby avoiding collisions between messages to be transmitted. Such a system is disclosed in JPO-A-57-81746.
Such a system is disadvantageous in that a high frequency of transmissions by a particular data transmission apparatus results in less availability of the loop transmission line for communication among other sets of apparatuses, and a correspondingly lower frequency of transmissions therebetween. Any attempt to transmit with a predetermined constant period encounters difficulty, as such a constant period is difficult to establish consistently without being disturbed.
Prior systems are not suited for cyclic communication such as is desirable for transmission of data at a constant period. Any difference in the frequency of transmission requests results in a disturbance in the periodicity of the transmission.
Another problem with the prior system is that it is not suited for a system in which a short transfer period is required. The priority assurance data which is circulated in previous systems also takes up considerble transmission time, thereby limiting the amount of useful data which may be transmitted over the system. In addition, more useful transmission time may be forfeited when explicit address information for memory interaction must be transmitted, thus further decreasing the efficiency rate of transmission of useful data.